This invention relates to the detection of flames with flame detectors that are sufficiently rugged to withstand extremely high temperatures, and yet sufficiently small in size to allow mounting in small areas (such as in aircraft compartments or engine nacelles).
Aircraft fires, if undetected and not suppressed, can cause catastrophic consequences. It is therefore extremely desirable to accurately detect fires that occur in, e.g., engine nacelles and storage compartments, and quickly suppress the flames. A detector should be sufficiently rugged to remain operational for some time even in the presence of extreme temperatures, so that as the crew takes steps to suppress a detected fire (e.g., by releasing chemical suppressants) the detector can continue to provide reliable information about the state of the fire so that the crew can make an informed decision as to whether to take more severe action (such as landing the aircraft immediately). In fact, one military specification requires aircraft flame detectors to remain functional when exposed to a temperature of 2000.degree. F. for 60 seconds. That same specification, however, as well as many other applications, requires the detector be sufficiently compact to be mounted within small, narrow spaces.
Many known detectors are housed in an anodized aluminum shell that has an outer diameter chosen to meet the size specifications. Encased within the housing is a flame sensor (e.g., a thermopile, pyroelectric, or photoresistive sensor) and a circuit card containing the detector electronics. To protect these components from shock and vibration, the remainder of the cavity is typically filled with a mechanically damping potting compound.
To increase tolerance to high temperatures, some detectors enclose the detector housing within a layer of thermally insulating potting material, which is itself surrounded by an additional outer shell (typically made from steel with a tin-plated outer surface). The added volume of material provides thermal insulation for the detector components mounted within the housing, but at the expense of increasing the size (i.e., the outer diameter) of the detector assembly.
To avoid needlessly expending chemical suppressants or unnecessarily landing and evacuating the aircraft, false alarms must be prevented. For example, the detector must distinguish between flames generated by chemicals such as hydrocarbons (to which a response is desired) and other sources of heat and light, such as ordinary sparking, sunlight, lightning, and fluorescent lighting. Additionally, it is desirable that the detector differentiates between brief flashes of hydrocarbon flames and sustained flames to which a response must be made.